The present invention relates to a method and apparatus for treatment of a wastewater containing organic matters. More particularly, the invention relates to a method of wastewater treatment and apparatus therefor when the wastewater contains organic matters hardly disposable by the methane fermentation digestion method and activated sludge method or when the wastewater contains a hardly decomposable or removable eutrophic substance such as nitrogen- and/or phosphorus-containing organic compounds.
As is known, treatment of wastewater containing hardly decomposable organic matters is conducted in the prior art mainly by the so-called activated sludge method, intermittent aeration digestion methods or denitrification method. These prior art methods, however, are each a kind of biological decomposition methods so that, when the wastewater to be treated contains a large amount of hardly decomposable organic matters, the wastewater must be diluted, prior to transfer to a treatment plant, by adding a large volume of fresh water resulting in a great increase of the wastewater volume to be treated, which leads to an increase in the electric power consumption and an increase in the cost for the process of wastewater treatment. In addition, these prior art methods are defective in respect of the low efficiency for the removal of nitrogen-containing matters reaching 50 to 80% at the highest.
With an object to overcome the above described difficulties, a method is proposed in the prior art in which a supernatant liquid from digestion of a wastewater containing hardly decomposable contaminant substances such as polyphenols is treated with ozone to oxidatively decompose the contaminant substances. This method is also not quite practical because the concentration of ozone must be high enough with an unavoidable increase in the treatment cost.
Besides, the method of contact aeration has a problem of a low efficiency for oxidation and the digestion method is defective because, although use of ozone in this method is effective, this method is applicable only to a sea water-based wastewater containing a relatively large amount of bromine ions since otherwise the amount of radical generation is very small not to complete denitrification.
On the other hand, it is known that oxygen radicals having a life of 10 to 100 xcexcs are generated in the pores found in the surface layer consisting of particles of titanium dioxide, tin oxide, iridium dioxide and the like when electrons enter the pores and the oxygen radicals have an activity of oxidatively decomposing carbon- and nitrogen-containing contaminant matters while the efficiency of oxygen radical generation can be improved by appropriately selecting the electric voltage between the electrodes, electric current passing between the electrodes, frequency of the electric field applied, dimensions of the pores on the metal oxide surface and the degree of cleanliness desired for the wastewater after treatment.
It is, however, the present status of the above proposed method of utilizing oxygen radicals that the method has not yet been rendered to practice for wastewater treatment.
The present invention has been completed with an object to provide, by utilizing oxygen radicals and hydroxyl radicals, to which no attention is directed heretofore, a novel and efficient method for the treatment of wastewater containing hardly decomposable contaminant substances and to provide an apparatus for practicing the method.
The inventor has continued extensive investigations on the behavior of oxygen radicals and hydroxyl radicals generated on the surface of a metal oxide and, as a result thereof, has arrived at a discovery that the efficiency for the generation of these radicals can be improved and sustain ability of the radicals can be increased when the electric field applied between electrodes for radical generation satisfies specific conditions and that the efficiency of wastewater treatment can be improved by increasing the contacting time between the wastewater and the metal oxide surface leading to completion of the present invention on the base of this discovery. Additionally, it has been discovered that, when the wastewater contains a large amount of suspended solid matters (SS), a remarkable improvement can be obtained in the efficiency of wastewater treatment by applying ultrasonic waves to the wastewater flowing between the electrodes presumably due to a cleaning effect on the electrode surfaces.
Namely, the present invention provides an improvement, in a method for wastewater treatment in which the wastewater containing organic matters and flowing through a wastewater channel is passed between a positive electrode having a surface made from a specific metal oxide and a negative electrode having a surface of a noble metal forming a radical generating zone so as to clarify the wastewater by oxidatively decomposing the organic matter contained therein with application of a direct-current voltage between the electrodes, which is characterized in that the flow channel of the wastewater has a cross sectional profile gradually increasing from the upstream to the downstream and the direct-current voltage applied between the electrodes has a voltage gradient of 0.2 to 6 kV/cm in a rectangular pulsed wave form at a frequency of 10 kHz to 150 kHz to give an electric current density of 5 xcexcxc3x85/cm2 to 50 mxc3x85/cm2 between the electrodes.
Further, the improvement comprises applying pulsed ultrasonic waves to the flowing wastewater in an ultrasonic zone located just downstream of the radical generating zone under such a time sequence control that the pulses of the direct-current voltage applied to the electrodes and the pulses of the ultrasonic waves are not overlapped in time sequence.
Further, the apparatus of the present invention for practicing the above described method of wastewater treatment is characterized in comprising a tubular conduit having, at one end, a radical generating zone consisting of a first truncated part with a side surface having a surface layer of a metal oxide to serve as a positive electrode and opening at the top flat to serve as a wastewater intake, the side surface of the truncated part making an angle of 5 to 40 degrees with the axis of the tubular part, and a negative electrode in the form of a rod having a surface layer of a noble metal and insulated from the positive electrode, the negative electrode being located along the axis of the tubular part and the positive electrode and the negative electrode being connected to a direct-current power source capable of applying a direct-current pulse voltage of a rectangular wave form at a frequency of 10 kHz to 150 kHz to the electrodes forming a voltage gradient of 0.2 to 6 kV/cm between the electrodes with an electric current density of 5 xcexcxc3x85/cm2 to 50 mxc3x85/cm2.
Further, the above described tubular conduit of the inventive apparatus has an ultrasonic zone consisting of a second truncated part opening at the top flat to serve as a wastewater outgo and connected to the tubular part at the other end opposite to the radical generating zone, the side surface of the second truncated part making an angle of 5 to 40 degrees with the axis of the tubular part and an ultrasonic vibrator being mounted on the second truncated part.
The method of the present invention for wastewater treatment is an electrochemical process utilizing oxygen radicals and hydroxyl radicals originating in water molecules. The hydroxyl radicals may react with a carbon source material generating carbon dioxide according to the following reaction equation:
C+2(OH)xc2x7xe2x86x92 CO2+2H+.
When the wastewater under treatment contains a large amount of suspended solid matters, there may be possible disadvantages of excessively large consumption of electric energy required for charging up the particles and deposition of a large amount of the particles in the pores of the metal oxide electrode resulting in a great decrease in the radical generating reaction. This disadvantage can be overcome by means of the ultrasonic waves applied to the ultrasonic zone of the tubular conduit.